An aluminum alloy is light-weight, has high thermal conductivity, and can realize high corrosion resistance with proper treatment. Therefore, the aluminum alloy is used for a vehicular heat exchanger, such as a radiator, a condenser, an evaporator, a heater, and an intercooler. As materials of tubes for vehicular heat exchangers, there are used a two-layer clad strip formed by employing an Al—Mn based alloy, e.g., a 3003 aluminum alloy, as a core, clad with a filler material made of an Al—Si based alloy or a sacrificial anode material made of an Al—Zn based alloy on one surface of the core, and a three-layer clad strip formed by further cladding the filler material made of the Al—Si based alloy on the other surface of the core. In the heat exchangers, such a clad strip and corrugated fins are usually combined and joined to each other by brazing them at high temperature of about 600° C.
When a corrosive liquid is present inside and/or outside a tube in the heat exchanger, there is a possibility that the tube is burst because the tube may thoroughly be holed due to pitting corrosion and because a wall thickness of the tube may be reduced due to uniform corrosion which causes reduction of compression strength (pressure capacity). This leads to a risk that air, cooling water or a coolant circulating inside the tube may leak. Hitherto, inner and outer surfaces of a tube in a radiator, for example, are exposed to corrosive environment because cooling water flows inside the tube and a corrosive substance coming from external environment, e.g., snowmelt salt, adheres to the outer side of the tube. To cope with such a problem, the inner side of the tube is made corrosion resistant by cladding a sacrificial anode material thereto. For the outer side of the tube, Zn is added, for example, to the fins in order to make a pitting potential of the fins relatively lower and to utilize the sacrificial anticorrosion action developed by the fines without cladding a sacrificial layer to the tube itself. The reason why the outer side of the tube can be made corrosion resistant by the above-mentioned method resides in that the corrosive liquid adhering to the outer side of the tube has high electric conductivity. Electrical conductivity of the corrosive liquid rises as the concentration of a solute component in the corrosive liquid increases. In the external environment of the radiator, the corrosive liquid containing the solute component, such as snowmelt salt, at a high concentration, adheres to the outer side of the tube, thus providing high electric conductivity. Accordingly, the entire tube can sufficiently be made corrosion resistant with the sacrificial effect of the fins.
In a new heat exchanger used in recent vehicles, however, it is required to employ a tube clad with sacrificial layers on both the inner and outer sides of the tube. One example of such a heat exchanger is a water-cooled intercooler. In the water-cooled intercooler, of inner and outer surfaces of a tube, the surface on one side contacting with cooling water is exposed to corrosive environment as in the conventional radiator, and therefore it requires cladding of a sacrificial anode material. On the other hand, the tube surface on the opposite side is contacted with compressed air mixed with exhaust gas. In that case, the compressed air cooled in the heat exchanger causes dewing of condensate water in which exhaust gas is dissolved. The condensate water contains a chloride ion that is a component of the exhaust gas, and hence has pitting inductivity. For that reason, sacrificial corrosion resistance is also required on the tube surface on the side exposed to the compressed air. However, because the solute component in the condensate water is rare and the tube is in environment not immersed in water, it is difficult to develop the sacrificial anticorrosion action by the fins. Accordingly, the tube surface on the side exposed to the compressed air is needed to have both a superior brazing function for the fins and the sacrificial anticorrosion function.
A tube illustrated in FIG. 2, for example, is formed in a tubular shape by brazing opposite ends of an aluminum alloy brazing sheet made up of three layers of filler material/core/sacrificial anode material to each other. The sacrificial anode material is coated on the inner surface side of the brazing sheet, and the filler material made of an Al—Si based alloy is coated on the outer surface side. One method for giving the sacrificial anticorrosion effect to the conventional filler material made of the Al—Si based alloy, in addition to the brazing function, is to add Zn to the filler material such that the pitting potential becomes relatively low. With that method, a filler metal melted from the Al—Si based alloy is collected and solidified in a joint portion during brazing. Because the Al—Si based alloy contains Zn, Zn is inevitably concentrated in a finally solidified portion. In such a state, the pitting potential of the joint portion becomes lowest and corrosion is preferentially generated in the joint portion. Because cooling water is filled in the tube of FIG. 2, the preferential corrosion progresses from one end of the joint portion to the other end as indicated by an arrow D in FIG. 2(a), and it eventually turns to a corroded portion as illustrated in FIG. 2(b).
Patent Documents 1 and 2 describe brazing sheets each including a clad layer that has both the brazing function and the sacrificial anticorrosion effect. Those brazing sheets are mainly used in a radiator in which cooling water flows inside a tube, aiming to assist brazing in a portion C of the tube that is formed into a B-like cross-sectional shape as illustrated in FIG. 3. In the brazing sheets according to those known techniques, however, the sacrificial anticorrosion with the tube itself is not taken into consideration for the outside of the tube, and the conventional filler material made of an Al—Si based alloy is just clad over the tube on the side exposed to outside air. Thus, in the above-cited Patent Documents, no considerations are paid to the preferential corrosion in the joint portion between tube materials.
In more detail, the brazing sheet described in Patent Document 1 enables the tube materials of B-shaped to be brazed. Composition of a clad layer is set so as to reduce a Si amount in comparison with that in the usual filler material made of the Al—Si based alloy, and to prevent reduction of the sacrificial anticorrosion effect due to drift of the melted filler metal. However, Patent Document 1 just states that an amount of Zn added to the filler material is not more than 7%. It does not take into account how to suppress concentration of Zn into the joint portion. Stated another way, prevention of the preferential corrosion in the tube joint portion is not dealt with in Patent Document 1.
On the other hand, a clad layer having both the brazing function and the sacrificial anticorrosion effect, described in Patent Document 2, is added with Cu and Mn in addition to Si and Zn. However, addition of Cu and Mn aims to increase strength instead of aiming to prevent the preferential corrosion in the tube joint portion. Furthermore, the sacrificial anticorrosion effect with the clad layer is targeted on the cooling-water side of the tube, and a usual filler alloy, e.g., a 4045 aluminum alloy or a 4343 aluminum alloy, is employed on the atmosphere side. Thus, anticorrosion effect by the clad layer of the tube on the atmosphere side is not taken into consideration.